The present invention relates to a process and apparatus for contacting two immiscible liquid phases, for example in the organic extraction of an aqueous phase. The invention has particular, but not exclusive, relevance to, and will be described with respect to, the preparation of peracids (by which we mean herein peroxycarboxylic acids). The use of such peracids is well known in the epoxidation of alkenes, especially lower alkenes. Those skilled in the art of liquid-liquid extraction will readily understand what other processes the present invention can be applied to.